Stretch forming machine with tension control



Feb. 9, 1960 z. s. FREDERICKS 2,924,261

STRETCH FORMING MACHINE WITH TENSION CONTROL Filed Dec. 30, 1957 4Sheets-Sheet 1 INVENTQR.

' ATTOENEK 1960 z. s. FREDERICKS 2,924,261

STRETCH FORMING MACHINE WITH TENSION CONTROL I Filed Dec. 30, 1957 4Sheets-Sheet 2 1 24 rm/MEX Feb. 9, 1960 Z. S. FREDERICKS STRETCH FORMINGMACHINE WITH TENSION CONTROL Filed Dec; :50. 1957 4 Sheets-Sheet 3ATTORNEY Feb. 9, 1960 z. s. FREDERICKS 2,924,261

STRETCH FORMING MACHINE WITH TENSION CONTROL Filed Dec. 30. 1957 4Sheets-Sheet 4 WRAP 5 INVENTQR.

STRETCH FORMING MACHINE TENSION CONTROL Zygmuut S. Fredericks,Chesterland, Ohio, assignor to Tthglgyril Bath Company, Solon, Ohio, acorporation o Application December 30,1957, Serial No. 705,940

8 Claims. (Cl. 153-40) This invention relates to a stretch formingmachine of the general type disclosed in US. Letters Patent No.2,514,830, issued July 11, 1950, to'Cyril Bath, and particularly to anew and improved stretch forming-mac'hine'and control. combinationbyvirtue of whi'ch'the tension applied to the stock during the formationthereof can be controlled and varied as desired as the forming of thestoc'k'progresses.

Heretofore, in stretch forming metal stock, the-custoin'has been toanchoro'ne portion of a length of stock in fixed position relative'tothe operating 'face-of-a side face die, to grip the stock at anotherportion by asuitable tensioning assemblage, usually a piston andcylinder assemblage, and to form the stock by m'oving-th'edie andassemblage in a direction to wrap the steak-progressively onto the sideface of the die while-maintaining thestock under tension near to theelas'ticlimit of theistok. Herei pressure. However, constant pressuresupplied the piston and-cylinder ass-emblagedoes not produce a constanttension nor a constant elongation'due to a l-'arge number of variablesinherent in Lie assemblage itself, These variables, such as the changesin fr-iction and ibinding stresses as the assemblage extends andretracts and-shifts k v in alignment laterally from its-startingposition a re complicated further by changes in the cross section andphysicals of the metal. in fact, to obtain constant-tension on thestock, the exact reverse 'i's-true, variablepressure, not constant, mustbe provided in the assemblage.

One prior attempt to obtain prope'r control 'w'a's to circumvent theuse, for control, of variable pressure and its inherent difficultiesand, instead, to provide continuous'ly sufiicient pressureinthepistonand cylinder assemblage to stretch the stock'far be'yond itselastic limit, or even to its breaking point, and-tl1en tomontrol themovement of the gripping head of the stretch forming assemblage by apositioning device. This-type secontrol means operates on the principlethatfregardless of the higher pressure applied by the head to thestoek,the head is stopped in position as soon as thes took has been stretcheda predetermined amount. The positioning device is related in operationto the forming of the stock by means of a tape which is wrappedabout-a-template having a curvilinear face substantially the sameincontour' as, and coaxial with, theface of thedi'e. "'Onee'n'd of thetape" is "secured for movement withthe template and the other is securedto' the'positioning device located on the part of the stretch'forminguasse'mblage to-which the stretching head is secured. The ipositioning fdevice operates a servo-valve which controls the directionandrate of flow of pressurefluid to and from the sides of the piston to'cause the-piston and cylinder assemblage to extend and retract inrelation to the extension and retractionof the positioning device asoperated by the tape. solution requires a -templatewhich -is pro.

' fastened in fixed-relation to'one-end-ofrthe temp1ate,.and.

2,924,261 Patented Feb. :9, .1960

ice

cisely shaped relative to the die, so that for each unit length of tapewhich is wrapped onto the template, an equal unit length of stretchedstock is wrapped onto the die face. Thus, regardless of the stretchingforce applied by the piston and cylinder assemblage and the resistanceof the stock being formed, the stock is elongated a very definite andpositive amount by linear dimensional control only. The relativemovement of the stretch forming head follows a predetermined accurate"dimensional path with respect to the die regardless of the tonnageapplied, of the cross sectional area of the stock, of thecharacteristics of the material, or of the personal characteristics ofthe machine operator. This positive position control-has certainadvantages and limitations.

Another control is one in which the elongation of'the stock 'iscontrolled by controlling the tensioning force applied to the stock, themeasurements of the tension applied being measured between the stock andthe-stretch forming assemblage so as to eliminate the various frictionaland binding'stresses and the like inherent in the assemblage itself. Inthis type of control, examples of which are described in copendingapplications of Henry W. Hein and Zygmunt S. Fredericks, Serial No.568,553., now abandoned, filedFebruary 29, 1956, and of Richard L..Curtner, SerialtNo. 667,376, now Patent No. 2,849,048 filed June 24,71957, all ofthe extraneous binding stresses and .thelike inherent inthe stretch forming assemblage and its position are combined with;the;force 'due :toxthe pressure fluid and only the resultant (singleforce .and stress due .to these: forces and that caused by the appliedpressure fluid pressure is measured and controlled. .Accordingly, thestress applied toithe stock canbe main? taihed .constant or'can bevaried, depending upon the desires of the operator.

In the .Curtneriapplication, the itension applied .to the stockiscontrolled by varyingthe pressure in :the high pressure sideofjtherstretch forming :assernblage by variable venting of the pressurefluid in -.the high ;pressure side of the piston-and.cylinderassemblage.The variable venting is by means .ofa'relief alve discharging itoa sump,and under the control of a motor device which, in turn,;is-operated inresponsetothechanges insaiload cellstrain produced on the load cellby'the single tre-v sultant force applied by the stockand assemblage,*the load cell being located between theassemblage and its stockgrippinghead .forthispurpose.

iln accordance with the present invention, the .elonga tion andtensioningof the stookiscontrolled also by varying the. pressure in thehigh pressure side :of the stretch forming piston and cylinderassemblage, but, instead of this .-pressure being variedinresponse'toiaload cellxsubje'cted tozthe tension applied 'tothe stockitself, it *is varied in response to ya loaducell operated-hyaitapeiwhich, in turn, is-wrapped-about atemplate concurrently with thewrapping of the stock about the :sideface vof the die.

In general, in case the .stretchforming machine is of the typedisc'losedinzthe above identified Bath paten t,. a rotatable die ismounted on a turntable and-the template is mounted with its axiscoincident with the axis of the side face of 'the die. One ndof a lengthoftape is the, jother end is fastened in I fixed relation *endwise ofzthe stretch forming assemblage relative Ito the gripping ahead of theassemblage which grips the stock. Thedoad cell isinterpose'd betweet-the 'latter-i endaof the:tapeand'the stretch forming assemblage, -so-"that it is strained-due to,

e r a g 2,924,261.

Thusthe present invention distinguishes from that of load cell iseffected by means of tension applied by a tape and a template or othermeans movable in fixed relation to the side face forming die,r ratherthan by the tension on the stock itself. 1 i r The present inventionhas'an advantage in that, by changing the template,'the tension applied.to the stock at different points along the length of thestock are beingprogressively laid onto the side face die, may be varied so that thetension at any instantaneous pointjof tangency of the stock and sideface die can be readily preselected. 1 Further, it has an advantage inthat, when the stock is laid onto the die and it is found that thefinished piece does not quite conform to that desired, thetension can bevaried by modifying the template slightly until exactly the right stressis obtained'for the particular piece.

The present invention also has great advantageswhen a wipe forming shoeis used inconnection with the stretch forming, or when a suitable clampsuch as described in the above Bath patent, is mounted on the table andholds the stock firmly into engagement with the side face of the stretchforming die, after a predetermined portion of the length of the stockhas been formed. In such instances, it is apparent that by holdingtheformedportion of the stock firmly against the side face die at theinstantaneous line of tangency, greater pressures can be applied at theunformed portion of the stock. Again, the tension can be increased ordecreased where desired, as the stock is progressively laid onto theside face of the die, depending upon the cross section of the metal,degree of curvature, and the like. 1

Further objects and advantages will become apparent from the followingdescription whereinreference is made to the drawings which illustate apreferred embodiment of my invention, and in which: I i

"and wipe form mg concurrently, and it has been the Curtnerapplicationprimarily in that the strain of the a Fig. l isa perspective view of amachine illustrating base 1 on which is mounted a rotatable turntable 2,a

stretch forming pistonand cylinder assemblage 3, and' a wipe formingpiston and cylinder assemblage '4. A

carriage 5 is mounted on slideways 6 on the frame 1 for movement towardand away from the table 2 to different adjusted positions. is a support7 which is rotatable about an upright axis Referring to the drawings,the machine comprises a- Mounted on the carriage 5,

desirable that a wipe forming piston and cylinder assemblage be arrangedfor reciprocation alongside the face of the stock being formed on thedie, in addition to its being'operative to force awipe forming shoetoward the face of the die. For this purpose, the base or frame \1 has alateral extension 20Twhich is located alongside the turntab1e 2,.andwhich .has guideways ,21--.-.on which a suitable saddle or carriage-22is, mounted for reciprocation alongside the table.- "'Ihewipe formingpiston and cylinder assemblage t4 isamounted on .the carriage i and hasa cylinder 23 in ifixed'positionon the carriage 22. The assemblage has apiston 24 having 'a rod 25, which is connected to a pedestal26ofafslide27. The

wipe forming shoe or'toot 28 is connected to the pee estal in a positionso 'asto be pressed laterally against the stock as it is being wrappedalong side, face die. If desired, the wipe forming cylinder may becontrolled by separate power means and fluid pressure means. in-

:by the stretch forming assemblage.3, with which the present inventionis particularly concerned, a load cell 32 is connected to the pedestal13 atone end so as to be in fixed relation thereto. The other end of theload cell 32 is connected to a tape 33the opposite end of which isconnected by a clamp 34 toa template 35. The templateis preferablyarranged so as to be supported above the die D with its operating faceas near 'as may be par.- allel to the operating face of the die D butoffset therefrom towards the axis of the turntablexZ. Thus, as theassemblage 3 and die D are moved relative to each other so as to wrapthe stock progressively about the dieface,

: the tape 33 is concurrently progressively wrapped about and whichsupportsa saddle 8 on which the assemblage it 3 is carried. Theassemblage 3 comprises a cylinder 10 which is mounted in fixed positionon the saddle 8 with its axis extending generally toward and away fromthe table in a plane normal to the table axis; A piston 11 having apiston rod 12 is mounted for reciprocation in the cylinder 10, and isfixedly connected at its outer end to a pedestal 13 of a slide 14 which,in turn, is mounted in guideways 15 of the saddle for movement parallelto the axis of the cylinder 10. Mounted in fixed position endwise of theassemblage 10 on the pedestalone end ofa length of stock S. The head 16is mounted on the pedestal for rocking movement about an-uprightdesirable to subject the, stock to both stretch forming the side face ofthe template 35.

In the type of machine illustrated, this relative movement is effectedby rotation of the turntable 2. Since the pressure maintained in=thepiston and cylinder asi semblage 3 is sufficient tostretch the stock, itis, of

course, greater than that which would'be necessary. to

elongate the load cell 32' by the relativelyweak tape.

However, as the tension on the loadcell 32 increases,

the means responsive to the. load cell are operative to. reduce thetension-and, as the tension decreases beyond connectedbetween the.pressure side of the. pump39 and the directional valve 37. Thedirectional valve is connected to opposite ends of the cylinder 10 bylines 41 and 42. a t t In one. position of the valve 37, the controlledhigh pressure fluid is supplied to the rod end of the cylinder 10through the line 41 .and concurrently the low pressure fluid is.supplied to the head end through the line42. Thus, the pull exerted onthe piston by the stoclcis yield-.

ably resisted by the high' pressure fluid, the unit pressure of which iscontrolled by the loadcell device 32, diminished to a' slight'extent .bythe. pressure exerted on the headwend of the piston by the lowpressurefluid, As the piston is extended, the excess volume of highpressure fluid is exhausted to sump from the rod end of the cylinder 10through the-1ine 41 and the high pressure relief valve 38,. :As thepiston is retracted the excess low pressure fluid exhausted to sumpfromthe headend and i 5 of the cylinder through the line 42audible-Impressure relief valve 40. a a

If it is desired to extend the piston the directional valve is reversedso that the low pressure fluid is directed through line 41 to the rodend of the cylinder '10 and the high pressure fluid is directed throughline 42 to the head end, thus extending the piston. The excess fluidin'the rod end ofthe cylinder 10 vents through the low pressure reliefvalve.

The differential between'high and low pressure is sutfieiently great atall times to effect this proper direction of movement selected.

The control of the pressure to the high pressure or rod end of thecylinder 10 is preferably effected by varying the setting of the highpressure relief valve 38. This valve is of the type described in theabove identified copending application, and is driven by a reversiblemotor 43 to different settings to increase and decrease the pressure atwhich the valve vents to sump. The motor '43 is controlled, in turn, bya suitable amplifier and control 44, which may be such as described inthe above application, tl1e signals therefor'being suppliedthroughsuitable electric lines 45 from strain gauges 46 arranged onthe'load cell 32. The load cell 32 is preferably of the I-beamconstruction but relatively light so as to be readilystrained by thetension applied by-the relatively Weak-metal tape 33. Thus, when thetension of the tape increases unduly,

the motor 43 operates the valve 31% to increase its venting and thusreduce its pressure, and, as the pressure decreases unduly, it cl'osesdown the venting ofthe valve 38 so as to increase the pressure. Thepressure supplied by the pump 36 is always considerablyabove thatrequired for the stretch forming operation. Accordingly, the tensionapplied to the stock is controlled by varying the pressure supplied tothe rod end of the cylinder 3 in response-to the tension exerted by'thetape 33 on the load cell 32, and the resultant strain of the load cell.

The permanent stretching of the stock adds a substan tial amount to itsfinal length, but the tension on the tape is insufiicient to cause acorresponding elongation thereof. Consequently, the len th or" the faceof the template 35 is made less than the length of the face of the die,inasmuch as it is required that a tension be continuously maintained onthe tape as the stock is laid on the'die, and to allow for thestretching and elongation of the stock-as it is laid on the die. 2 I

in order to understand the operation and how it distinguishes from apositional controL-reference is made to Fig. 5, wherein the relativelengths of the tape and stock and the distance of the stock driving-headand the load cell from their starting points, measured along the 'tem-'plate and die faces, respectively, we shown by diagrams in a number ofoperating positions.

The first position, indicated by diagram 50, shows the tape-33 asconnected at its right end to the template 35 and the stock S asconnected at its right end'to the die D. It is-to-be notedthat thelength L of the tape 33 is greater than the length of the stock S, asthe stock will be stretched prior to any rotation of the table 2. Theamount of elongation which the stock must have to make the finished partcan be determined readily in advance withone 'or two test pieces. Thiselongation is constant for subsequent lengths of stock for the samearticle.

Accordingly, the length of the stock measured between 7 its -point ofconnection with the 'die and thestretching head-is equal to L minus aconstant, or L minus c, in the starting position. However, beforerotation of thetable, when the stretching fluid pressure is introducedinto the rod end of the cylinder :10 for 'jprestr'etching the stock,-the stock length is increased by an amount s, as indicated in diagram51. When the amount of stretch s is sufficient so that the length of thestock and the tape are equalLthe stretching force is reduced by thecontrol action of the tape, so as not to cause further stretching afterthe initial stretch takes the slack out of vth'extape and unwrappedportion of Lx. Since the length of the die face is greater thanthelength of the template, in that the template face is offset towardthe axis of the turntable I from'the die face'at all points along itslength, the wrapping of an amount of tape equalto length x results inthe wrapping of an amount of stock equal to a length x-I-Ax, wherein Axis the incrementby which the stockS- is elongated over and above itsinitial stretch or elonga tion s.- This leaves, as the unwrappedremainder of -the stock, a length equalto Lx, as illustrated in diagram52. As the wrapping continues and the additional length y of the tape 33is wrapped on the template, leaving an un- Wrapped length of -Lxy, thelength of the stock wrappedon the die is equal to -x-l-y-l-Ax-l-Ay, 'Aybeing the additional stretch of the piece of stock leaving an unwrappedportion of the stock equal to L-x y, the same asfor the tape, asindicated in diagram '53.

Attheend of the wrapping operatiorn'as indicated in diagram '54, all ofthe tape of length L having been wrapped, as indicated successively bythe lengths x-l-y-l-r, the stock also has been fully wrapped and thewrapped portion is equal to 'x-l nx+ v+Ay+z+Az, in which or and Ay andor arethe increments of-stretch imparted to the stock S in addition tothe prestretch s originally im-' parted to the StQC-kyZS indicated at sin Fig. 1.

Sometimesa constant tension is maintained on the stock throughout theforming operation. However, 'in some instances the stock, at differentlocations along its length,: has difierent cross sections or differentdegrees-of bend or indentation both later-ally of the die face andendwise of the die face. In other instances, the stock is wider at oneportion than at another. As aresult, it is desirable to vary the totalstretching force to maintain the desired unit force for forming theparticular portions.

Accordingly, the face of the template does not 'neces-' sarily have toremain at all times parallel to,'or offset the same distance inwardlyfrom the face of the 'dieto ward the axis of the turntable or in thedirection in which the'stock is moved to approach the die face."Instead, its contour'may besuchas to'approachandrecede" from the faceof the die different amounts at different portions along its length so"that the pressure can be varied in accordance with-the demands ofstock.

.For example, it may be desired in Fig. 5 that the por-- tion x bestressed at a degree different fromtheportion'y] of the stock. If, for aunit length along the die "face, the length of the templatecorresponding to that'uni t' length is increased, more tape iswound-onto .the'template and this increases the strain on the load cell,causing a reduction in pressure fluid pressure.

apparent that, with this arrangemenhthetension applied" to the stock canrange from the maximum of whichthe' hydraulic system is capable down tozero.

These variations in pressure are particularly desirable when the stockis concurrently wipe formed and stretch formed, as the wipe shoe itselfaffects'the tensionon the 7 stock. 1 r

If extreme cases so require, one or more holding piston and cylinderassemblages may be provided on -the table, as described in the aboveidentified Bath-patent As illustrated in Fig. 3; a holding pistonandcylinder On the other hand, if, for a unit length of the die face,'theface of the template is decreased correspondingly, then less tape"81 a a that the length of the template face is less than'the length ofthe die face. i i 3. The structure according to claim 1 characterized inthat the ratio of the length of the template face, measured fromthe"starting positionof the tape to the instantaneous linejof tangencyof thetape and template, and the length of the die face; measured from thestarting position of to which different portions of the length of stockare subjected. .For example, beginning with the stock at theright-handend of the die being stretched under light pressure, it may bedesirable beyond the point at which the clamp 58 .is positioned togreatly increase the pressure. In such a case, the clamp 58 is appliedjust beyond the instantaneous point of tangency in a direction towardthe starting end of the die, and is held firmly clamped to the dieduring continued rotation of the table. During this continued rotation,the tension is greatly increased between the clamp 58 and the stretchforming head 16. In such an instance, the face of the template isfarther removed from the face of the dieD at the location of the clamp58 so that for a given unit length of the die, the length of thetemplate is still further decreased than normal. This results inslackening the tape and therefore a demand for greater tension.Thetension is increased until the stock is stretched enough to removethe slack in the tape. This increase continues to the point desired. Ifit is.desired to reduce the tension,

the face of the template is shaped to approachrnore closely to the faceof the die, thus increasing the tension;

on the tape and calling for a reduction in pressure on the stock whichwould thenbe subjected to lesstension. It is apparent that, with thepresent structure, a wide the stock to the instantaneous line oftangency of the stock and die, is different for different locations ofthe point Qf tangencyof the die and stock. p e

4.,The structure according to claim 1 characterized in thatthe distancebetween the faces ofthe die and template, in said directionofrelativemovement, is different at ditferentjlocations endwise of the dieface, whereby the stock is subjected to different preselected tension atdif erent-portions along its length as it is wrapped onto the die. a t

;5 The structure'according to claim .1 characterized in that said load:cell, is strainable for operation at tension lessthan thatrequired toimpart a permanent stretch to the'stock.

t 6. The structure according toclaim 1 characterized in.

that the tensioning means is a piston and cylinder assema blageconnected in a circuit to a source of pressure fluid, the settablemeansisa variable relief valve and is arranged in said circuit betweenthe source and the pressure side. ofthe cylinder so as tochangethetension applied variety of effects in combined wipe and stretchformingcan be obtained and all are within ent invention. j M Having thus,described my invention, I, claimzw t 1. A stretch forming machine .forstretch forming a length of metal stock against a curved side facediepro gressively from one, location to another on the face the scopeofthepres:

whilethe stock is held under tension endwise, including.

a curved side face die, means adapted to securethe stock at one portionin fixed position relative to the die, tool means. adapted to beconnectedto; the stockfat; another portionfor transmitting'endwisetensioning force to the. stock, tensioning means connected to the toolfor applying said tensioning force to the tool, settable'm'eans settablein different operating positions for controlling:

the tensioning means and thereby the tension, applied to the stock;means supporting the die and tensioning means forrelative movement inadirection transversely of the die for wrapping .the stock onto thediezface progressively endwise from one'location on the face .to.another, power means for eifecting said relativemovement, a load cell,having substantialresistance to. elongation by tension 1 appliedthereto, connected to the: as-

semblage for endwise movement therewithin fixed endwise relation to thetool, reversible motor means connected to the settable means andoperative, when" energized, for setting the settable means, strainsensing means carried by the load cell so that changes in the; strain'ofthe load all produce related changesin the conditioripf the sensingmeans, and said sensing means beingiopera-u tively connectedto the motorfor etfectingqoperation of thegmotor in opposite directions as thestrain on the: cell increases and decreases, respectively, a side facetern-z;

' by'the cylinder.

7. A stretch forming machine for stretch forming a length .of metalstock-against a curved side face die progressively from onelocationtoanother on the face while the" stock. is held under tension endwise,including a curved side face die, means adapted to secure the stock at,one portionin fixed position relative to the die, tool means adapted tobe connected to the stock at another portion for transmitting endwisetensioning force to the, stock, a fluid pressure operated piston andcylinder assemblage connected to the tool for applying said tensioningforce to. the tool in relationqto the pressure ,of working pressurefluid supplied to the assemblage, a fluid circuit for supplying saidpressure fluid from a source to the. assemblage, a variable relief valvemeans in the. circuit and settable in different operating positions forcontrolling the pressure of the operating fluid supplied to theassemblage, means supporting the die and assemblage forrelative'movement in a direction transversely-of the die forwrapping thestock onto the die face progressively endwise from one location on theface to another, power.

meansffor effecting said relative movement, a load cell,

havinga substantial resistance to elongation by tension applied thereto,connected to. the assemblage for endwise. movementtherewith in fixedendwise relation. to the tool, reversiblemotor means connected tothevalve and operative; when energized, for setting the: valve,,straingauge 'means mechanically connected to the load cell so that changes inthe strain of the load produce related changes in the condition ofthestrain gauges, and oper- ,atively connected to the motor for effectingoperation;

of the motor in oppositedirections. as the strain increases anddecreases, respectively,- atension tape, capable of exertingsufiicienttension to elongate said load cell, connected at one portionto theload cell, means connected to the tape at another portion andmovable relative to the load cell and in predetermined relation ,to the.die

for causing the tape toapply tension to the load cell V ina-predetermined relation to the tension applied to plate in-fixedposition relative to the die, a tensionutape,

capable of'exerting suflicient tension to elongate said'load cell,connected atone portion in fixedrelation to the template and at anotherportion'in fixed relation to the, load: cell for wrapping of the tapeonto the template face during.

said relative movement of the 'dieand assemblage,;so;as

applied by the tape.

- 2;. The structure according to. claim; 1 characterized in to strainthe'load celland in proportion to the tension l the stoek by theassemblage.

8. A stretch forming machine for-stretch forming a length of metal'stockagainst a curved side face die apro-,

gressively from one location to another onthe face while the stockisiheldmnder tension endwise, and including a ourvedside: face. die;means adapted to secure the stock at one portion in fixed positionrelative to the die, tool" means adapted to be connected. to :the stockat another portion fortransmitting endwise tensioning force to thestock, tensioning means connected to the tool for applying saidtensioning force to the tool, settable means for controlling thetensioning means and thereby the tension applied to the stock, meanssupporting the die and tensioning means for relative movement in adirection transversely of the die for wrapping the stock onto the dieface progressively endwise from one location on the face to another,power means for effecting said relative movement, reversible motor meansconnected to the settable means and operative, when energized, forsetting the settable means, tension sensing means, having substantialresistance to elongation by tension applied thereto, connected to theassemblage for endwise movement therewith in fixed endwise relation tothe tool, and connected to the motor for effecting operation of themotor in opposite directions as the tension on the tension sensing meansincreases and decreases, respectively, clamping means movable with thedie and operable for clamping the formed stock against the die in fixedposition at preselected locations, respectively, so as to isolate thetension applied to the unforrned portion of the stock from the formedportion, a side face template in fixed position relative to the die, atension tape, capable of exerting sufficient tension to elongate saidload cell, connected at one portion in fixed relation to the templateand at another portion in fixed relation to the tension sensing meansfor wrapping the tape onto the template face during said relativemovement of the die and assemblage, the length of the template facebeing less than the length of the die face so as to strain the tensionsensing means by, and in proportion to the tension applied by the tapeduring stretch forming, and the ratio of the length of the templateface, measured from the starting position of the tape to theinstantaneous line of tangency of the tape and template and the lengthof the die face measured from the starting position of the stock to itsinstantaneous line of tangency, being different for different locationsof the point of tangency of the die and stock.

References Cited in the file of this patent UNITED STATES PATENTS GrayFeb. 25, 1958 Bath July 11, 1950

